Hot air circulation apparatus and method for wave soldering machines

Abstract: A hot air circulation apparatus and method of heating for use in a wave soldering machine which heats air from a high-pressure air source and directs it at the flux applied to a printed circuit board. The heated and pressurized air also circulates air within the wave soldering machine to force the moisture out. A plurality of hot air knives are mounted adjacent heaters within the wave soldering machine and are coupled to the pressurized air source. The hot air knives include a hollow metal cylindrical member which conducts heat from the heater, and contain a plurality of orifices which increase the pressure of the air and direct the air at the flux.

Description: The present invention relates to soldering and soldering machines, and more specifically to a hot air circulation apparatus and method of heating for use in a wave soldering machine. Wave soldering machines are typically used to solder components onto printed circuit boards. Such machines employ a fluxer for applying flux to the solderable surfaces of the printed circuit boards, preheaters to preheat the printed circuit boards, and a solder wave to apply liquid solder to the solderable surfaces. The preferred flux used in wave soldering machines is a resin flux. However resin fluxes contain environmentally harmful and flammable volatile organic compounds. An alternative to resin fluxes is a water-based flux, such as the 970 and 970CTX flux products from Kester Corporation. An air knife is typically located after the flux tank to remove excess flux from printed circuit boards and prevent dripping on preheater surfaces. A warm gentle airflow is required so as not to blow the flux off the board. The water-based flux is environmentally safe, but when used in wave soldering machines suffers from the disadvantage that the water cools the printed circuit board and causes solder balls to form. Previous attempts at making the water-based flux usable in wave soldering machines have been unsuccessful. In EP96967, Cooper Industries Inc., a wave soldering device is provided with a high velocity heated gas stream for removing excess solder from a printed circuit board, but does not address the problem of removing water from water-based fluxes. It is accordingly an object of the present invention to provide a hot air circulation apparatus and method for wave soldering machines which is suitable for use with water based flux. According to a first aspect of the present invention there is provided a wave soldering machine comprising conveyer means for conveying printed circuit boards past a fluxer and solder - 2 application means, characterized by a hot air circulation apparatus for drying water-based flux on said printed circuit boards, said apparatus comprising a pressurized air source; and means coupled to the pressurized air source for heating the pressurized air and directing heated air towards the printed circuit board, said apparatus being located upstream of the solder application means. According to a second aspect of the present invention there is provided a method of removing moisture from a water-based flux on a printed circuit board moving through a wave soldering machine, comprising the steps of: providing a pressurized air source for producing pressurized air; heating the pressurized air; and directing the heated pressurized air towards said printed circuit boards to remove said water-based flux. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic view of a wave soldering machine employing the apparatus of the present invention; and Fig. 2 is a perspective view of an air knife. Referring now to Fig. 1, a wave soldering machine 10 employing the hot air circulation apparatus 12 of the present invention is shown. The wave soldering machine 10 also includes tunnel chamber 14, conveyor 16, fluxer 20, heaters 22 and 24, and solder pot 26. Conveyor 16 carries printed circuit boards 18 through tunnel chamber 14. Printed circuit boards 18 have components mounted thereon whose electrical contacts have previously been coated with solder bumps. Preferably, the flux applied by fluxer 20 is number 970 or 970CTX from Kester Corporation. Number 970CTX includes a special wetting agent to allow the flux to spread evenly. Heaters 22 and 24 preheat printed circuit boards 18 as they pass overhead. Solder pot 26 applies liquid solder and removes any excess flux from printed circuit board 18 and its components. Hot air circulation apparatus 12 includes hot air knives 28, connectors 29, hoses 32, and high-pressure air source 30. Hot air knives 28 apply high pressure, high-temperature air to dry the flux and to drive the moisture from the flux out of tunnel chamber 14. Hot air knives 28 are preferably mounted on heaters 22 and 24, thereby making installing and retrofitting hot air circulation apparatus easy. However, other mounting configurations for hot air knives 28 are envisioned by the present invention, which may include additional heaters. Preferably, hot air knives 28 are made of aluminum so as to conduct heat rapidly from heaters 22 and 24 to the high-pressure air rushing through knives 28. The first group of hot air knives 28 on heaters 22 tends to dry the flux, while the second group of knives on heater 24 tends to circulate and dry the air in tunnel chamber 14. Four hot air knives 28 on each of heaters 22 and 24 are preferred. Hot air knives couple to high-pressure air source 30 through hoses 32 and connectors 29. Ordinary high-pressure air connectors and hoses may be employed. Preferably, high-pressure air source provides about 62Ncm -2 (90 psi), which ensures adequate drying and circulation. Turning now to Fig. 2, hot air knife 28 is shown in more detail. Air knife 28 is simply constructed, starting with a metal pipe, preferably aluminium. A 1.02 cm (.400 inch) outside diameter pipe with a thickness of about 0.18 cm (.070 inches) and a length of about 61 cm (24 inches) is preferred. A standard end cap 46 is screwed onto end 44. At the other end 40 knife 28 has inside threads 42 for receiving connector 29. With both ends 40 and 44 sealed, air from hot air source 30 passes through orifices 48, which are evenly spaced and oriented in a line along surface 50 of the knife 28, perpendicular to the direction of travel of printed circuit board 18. Preferably, orifices 48 are about 0.76cm (.300 inches) on center apart, with the first orifice being about 3.8cm (1.5 inches) from end 40. Orifices 48 are preferably 0.13 cm (.05 inches) in diameter for about 62 Ncm -2 (90 psi). For this orifice diameter and input pressure adequate circulation pressure and air flow volume are provided to drive moisture from tunnel chamber 14.

Claims: A wave soldering machine (10) comprising conveyer means (16) for conveying printed circuit boards (18) past a fluxer (20) and solder application means (26), characterized by a hot air circulation apparatus (12) for drying water-based flux on said printed circuit boards (18), said apparatus comprising a pressurized air source (30); and means (22,24,28,29,32) coupled to the pressurized air source (30) for heating the pressurized air and directing heated air towards the printed circuit board (18), said apparatus (12) being located upstream of the solder application means (26). An apparatus according to claim 1, characterized in that the device (22,24,28,29,32) also circulates air within the wave soldering machine (10) and forces moisture from the wave soldering machine (10) when in use. An apparatus according to claim 1 or claim 2, characterized in that the device (22,24,28,29,32) comprises an air knife (28,29) comprising a hollow member sealed at one end (44) and coupled to the pressurized air source (30), and having a plurality of orifices (48) through which the pressurized air is expelled and which increase the pressure of the pressurized air and a heater (22,24) which heats said air knife (28,29). An apparatus according to claim 3, characterized in that said orifices (48) are arranged in a line (50) facing the printed circuit board (18) and perpendicular to the plane (16) containing said printed circuit boards (18), when in use. An apparatus according to claim 4 or claim 5, characterized in that said device (22,24,28,29,32) comprise a plurality of additional hot air knives (28) adjacent the heater (24) which together with said hot air knife (28) form a first group of hot air knives. An apparatus according to claim 5, characterized in that said device (22,24,28,29,32) further comprises an additional heater (22) and an additional group of hot air knives (29) adjacent said additional heater (22). An apparatus according to claim 6, characterized in that said first group of hot air knives (28) is arranged to substantially dry the water-based flux, said additional group of hot air knives (29) being arranged to substantially force the moisture from the wave soldering machine (10), when in use. An apparatus according to claims 3 to 7, characterized in that each hot air knife (28,29) is located on top of its respective heater (22,24) and below the plane (16) containing said printed circuit board (18), when in use. A method of removing moisture from a water-based flux on a printed circuit board (18) moving through a wave soldering machine (10), comprising the steps of: providing a pressurized air source (30) for producing pressurized air; heating the pressurized air; and directing the heated pressurized air towards said printed circuit boards (18) to remove said water-based flux.

Inventors: GIBSON DAVID EARL, GIBSON, DAVID EARL
Applicants: NCR INT INC, NCR INTERNATIONAL, INC.
Application year: 1994
Main classification: B23K1
IPC codes: B23K3, H05K3, B23K1